Heretofore in wells where two or more formations are producing into a single interior pipe and the fluids from all the formations are co-mingled in the pipe for production to the earth's surface, it has been difficult to test, e.g. pressure test, a single formation. This is so because it is difficult to isolate one formation so that the test results are reliably attributable to that formation and no other formation in the well and to do so without interrupting production from the other formations.